直径为d的实心圆轴受扭,若使扭转角减小一半,圆轴的直径需变为( )。
A A.<img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251114/23827156_0.png"> B
B.<img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251114/23827156_0_1.png"> C
C.0.5d D
D.2d